1. Field of the Invention
The present invention relates to a hydraulic control device of an excavator, and more particularly to a hydraulic control device for improving loading performance in case of a loading-on-truck on a slope by a combined operation of boom-raising and swing.
2. Description of the Related Art
The hydraulic excavator performs ‘front works’, such as digging and ground leveling, by the operations of ‘front-work-devices’ such as a bucket, an arm, a boom and the like, or performs ‘combined operation’ by simultaneous operations of the front-work-devices and a upper swing body, such as loading-on-truck.
As the hydraulic excavator commonly carry out digging on level ground, a hydraulic control device of the hydraulic excavator is usually designed in such a manner that the boom operation is superior to others. As a consequence, the boom operation, particularly boom-raising usually responds more quickly than the swing motion.
The afore-mentioned phenomenon will be described with reference to FIG. 1, which shows a conventional hydraulic control system of the hydraulic excavator. The hydraulic control system includes a first hydraulic pump 101 and a second hydraulic pump 102. A swing control valve 106 and a boom high-speed control valve 107 are connected to a hydraulic supply line 103 of the first hydraulic pump 101 in parallel through each parallel lines 105,106, whereas a boom low-speed control valve 111 and an arm high-speed control valve 116 are connected in parallel to a hydraulic supply line 104 of the second hydraulic pump 102 through each parallel lines 112, 113. Outlet ports of the boom high-speed control valve 107 and the boom low-speed control valve 111 are connected to boom cylinders 115 through a confluence line 120.
When the boom low-speed control valve 111 is shifted alone, only one of two hydraulic pumps, i.e., the second hydraulic pump 102 supplies its flow rate to the boom cylinders so as to operate them in a low speed. When the boom low-speed control valve 111 and the boom high-speed control valve 107 are all shifted, the flow rate discharged from both of two hydraulic pumps 101, 102 are confluent in the confluence line 120 and then supplied to the boom cylinders 115 so as to operate the boom in a relatively high speed. Reference numeral 118 designates an arm cylinder and reference numeral 117 represents an arm low-speed control valve.
Taking into account that the hydraulic excavators to perform digging on level ground, the hydraulic control system shown and set forth above is designed to ensure that the boom operating speed becomes faster than the swing speed in case of combined operation of boom-raising and swing. In other words, when the combined operation of boom-raising and swing is carried out on level ground, flow rate of a hydraulic pump is supplied to the boom cylinder more than the swing motor, so that boom operating pressure becomes higher than the swing operation pressure as shown FIG. 2. As a result, the swing operation is carried out only with torque corresponding to the boom operating pressure.
With the conventional hydraulic control system described above, however, in case that the combined operation of boom-raising and swing, e.g., loading-on-truck operations is performed on a slope, swing operation substantially does not occur until rest-inertia of the boom get decreased at the beginning of the combined operation of boom-raising and swing. This causes a problem that the loading-on-truck operations cannot be smoothly performed.